Geologic Map of the Seboyeta 7

Geologic Map of the Seboyeta 7.5′ Quadrangle, Cibola County, New Mexico

by

Steven J. Skotnicki, Paul G. Drakos, Fraser Goff, Cathy J. Goff, and Jim Risterer

New Mexico Bureau of Geology and Mineral Resources

INTRODUCTION The Seboyeta quadrangle rests about 35 miles east of Grants, New Mexico, north of the Laguna Pueblo. The village of Paguate resides hear the southeast part of the map. The area also encompasses a portion of the Cebolleta Land Grant. The oldest rocks in the study area are Cretaceous. They include thick deposits of dark marine shale, interbedded with tan-colored beach and barrier-island sandstone and coal and sandstone units deposited in a marshy river system. These sediments record the transgression and regression of the ocean along the Cretaceous Western Interior seaway between about 84 and 99 million years ago (Molenaar, 1974). During the late Cretaceous/early Tertiary Laramide orogeny the region was uplifted and eroded, gently tilting the Cretaceous layers to the northeast by about 2-3° and creating a gently south-dipping peneplain (or pediment). Between about 3.3 and 1.2 Ma volcanic rocks of the Mount Taylor volcanic field erupted across this surface (Perry et al., 1990). The earliest rocks included rhyolite and trachydacite, but most of the eruptive products on the mesas are variations of basalt. The alignment of cinder cones in the Seboyeta quadrangle suggests they erupted along a buried fissure that is oriented northeast-southwest. For a good overview of the geology of the Mount Taylor volcanic field see the New Mexico Bureau of Geology’s link at http://geoinfo.nmt.edu/tour/landmarks/mt_taylor/home.html.

ACKNOWLEDGEMENTS We thank the staff of Laguna Pueblo for allowing us access to their lands within the Seboyeta quadrangle. We are grateful to Lee Maestas, President, Cebolleta Land Grant, for allowing us to perform this study on the Land Grant and for access to Chupadero and Seboyatita mesas. Specific thanks go to Jed Elrod, Ranch Manger, Silver Dollar Ranch, and Robert Alexander, Range Specialist, Laguna Pueblo. Phil Kyle (New Mexico Tech) kindly spent time with Fraser and Cathy Goff examining the Seboyeta Canyon basaltic vent. Dave Mann of High Mesa Petrographics, Los Alamos, made our thin sections. The State Map Program, jointly funded by the U.S. Geological Survey and the New Mexico Bureau of Geology and Mineral Resources, supported this project. PREVIOUS WORK Dane (19??) made detailed studies of the Mancos Shale in the eastern San Juan basin and was able to make correlations with other upper Cretaceous strata in northeastern New Mexico and eastern Colorado. Moench (1963a, b) mapped the Seboyeta 7.5’ quadrangle and the Laguna 7.5’ quadrangle to the south. Schlee and Moench (1963a, b) mapped the Mesita 7.5’ quadrangle to the southeast and the Moquino 7.5’ quadrangle to the east. Lipman and others (1979) made a 1:24,000 scale geologic map of the Mount Taylor quadrangle to the west, which was remapped by Osburn and others (2008). Additional quadrangles in the Mount Taylor volcano area have been recently completed by Goff and others (2008; 2010) and McCraw and others (2009).

SOME SPANISH WORD DERIVATIONS Celosa – Jealous, suspicious, zealous (female) Chivato – Young goat (male) Chupadero – Anything that serves to pacify Cubero – Cask or tub maker (?) Encinal – Evergreen oak place Prieta – Dark, black (female) Seboyeta – Is it Anglicized from onions (Cebolleta) or is it related to fat (Sebo)? CRETACEOUS ROCKS Mancos Shale, Satan Tongue The Satan tongue of the Mancos Shale is exposed only in the northeast part of the map along the northern part of Seboyeta Canyon. Because it is high above the canyon floor, and below the steep cliffs of volcanic rocks, it is difficult to access. There are only a few good exposures and each requires quite a climb to get to. Measured sections 1 and 20 show the stratigraphy of two good exposures. In each, the unit actually contains quite a few sandstone layers up to 3 and 7 meters thick. These sandstone beds are mostly fine-grained quartz, are thin- to medium-bedded, and commonly show faint planar cross-beds in sets up to 50 cm. Photo 1 shows the thickest of the sandstone layers near the base of the unit (see measured section 20 for the location of the photo). The interbedded shale is dark gray and poorly exposed on slopes between the sandstone beds. Point Lookout Sandstone, Hasta Tongue The Point Lookout Sandstone forms the stratigraphically uppermost light gray sandstone cliff visible in the study area (Photos 2 and 3). Also, this unit is also only exposed in the northeast along Seboyeta Canyon, and exposures are not very accessible. It is a light gray, fine-grained quartz sandstone and contains rare darker lithic grains. The uppermost 5m shows planar cross-bedding in sets up to 1 m thick. Below the cross-bedded layers, bedding is mostly horizontal with some low-angle cross-beds, particularly in the lowermost 2-3 meters. Gibson Coal member, Crevasse Canyon Formation The Gibson Coal member is poorly exposed, except in two exceptional exposures on the west and south sides of Mesa Chivato (measured sections 2 and 20, respectively). The unit is composed of thick accumulations of shale that commonly contain dark brown and black layers of lignite coal up to about 2 meters thick or more. The coal is commonly interbedded on the millimeter to centimeter scale with lighter gray siltstone beds that give some exposures a weakly laminated appearance. Nearly all of the shale beds crumble easily and form slopes. Interbedded with the shale are several fine-grained quartz sandstone layers (Photos 4 through 6). Most are less than 1 meter thick, though some crumbly sandstone beds are up to 5 meters thick. Most sandstone beds are massive and show horizontal burrows along bedding planes, suggesting that these beds may have been bioturbated. Ripple marks are locally visible along bedding planes. Well-preserved plant impressions are visible in the upper half of the unit west of the mesa (Photos 7 through 9).

Dalton Sandstone member, Crevasse Canyon Formation The Dalton Sandstone looks very similar to the Point Lookout Sandstone. It is a fine-grained quartz sandstone that contains minor dark lithic grains. Some beds appear massive to very weakly bedded. Other beds show planar cross-bedding in sets up to about 50 cm thick, particularly the uppermost 8 meters. The Dalton Sandstone forms a prominent light gray cliff. On the top of the cliff in measured section 20 is a thin orange-colored massive bed that contains what appears to be wood impressions (Photo 10). It is not clear if this bed is part of the Dalton Sandstone or the base of the Gibson Coal member. Below the base of the Dalton Sandstone the upper part of the Mancos Shale, Mulatto tongue, is interbedded with abundant thin- to medium-bedded fine-grained sandstone beds (Photos 11 and 12). Mancos Shale, Mulatto Tongue The majority of the Mulatto tongue of the Mancos Shale is dominated by quartz siltstone and very fine-grained platy beds of quartz sandstone. The Mulatto is characterized by an overall tan to ocher color. Shale is actually a very minor component of this unit. From a distance the unit is characteristically a muted pale greenish orange, which is distinct from the overlying lighter gray Dalton Sandstone and the underlying slope-forming Dilco Coal member. The Mulatto tongue contains at least two intervals that are dominated by thinly-bedded very fine-grained quartz sandstone. These intervals commonly form cliff ledges that show no distinct cliff-forming beds (Photo 13). Many of the thin platy beds throughout the unit contain molds and casts of brachiopods, the large bivalve Inoceramus, and darker shell fragments that resemble shell debris and echinoderm plates (Photos 14 and 15). Dilco Coal member, Crevasse Canyon Formation In the study area the Dilco Coal member is characterized by interbedded shale and thin sandstone layers. The shale beds commonly contain brown to black coal (Photos 16 and 17). The base of the Dilco coal member is characterized in most areas by a basal mottled sandstone layer 1-2 meters thick. This basal sandstone contains abundant horizontal tubular burrows up to about 1 cm in diameter. The burrows are filled with sandstone identical to non-burrowed areas, but for some reason are slightly more resistant than the surrounding matrix and stand out in relief, especially on the bottom of beds. Hematite preferentially stains certain areas within the sandstone and, together with the burrowed regions, gives the sandstone layer a mottled appearance, rusty orange color. The shale that immediately overlies this basal sandstone layer has eroded back, in places providing the basal sandstone layer with a resistant cap resting directly on top of the underlying Gallup sandstone. Some workers have lumped this basal layer into the Gallup Sandstone, which has led to different interpretations as to where to place the contact between the Gallup Sandstone and the Dilco Coal member. In this map and report, the contact is placed at the base of the basal sandstone layer. Overlying the basal sandstone layer is a sequence of interbedded dark gray shale and very finegrained yellow sandstone. The sandstone layers are typically between 0.5 and 2 meters thick, while the shale intervals are commonly between 3 and 7 meters thick. The interbedded sandstones are fine- to very fine-grained quartz, are thin- (60 meters.

Kph

Point Lookout Sandstone, Hasta tongue. Fine-grained quartz sandstone with rare darker lithic grains. Uppermost 5 meters shows planar cross-bedding in sets up to 1 m. Below about 5 meters bedding is mostly horizontal with low-angle cross-beds, especially in the lowermost 2-3 meters. Forms prominent light gray cliff. Thickness 45 meters.

Kcg

Gibson Coal member, Crevasse Canyon Formation. Interbedded light orange very fine-grained quartz sandstone in massive to thinly bedded layers up to 4 meters thick and dark shale. The shale commonly contains dark brown to black lignite coal in seams up to 2 meters thick. Locally contains light gray fragments of fossilized wood. Thickness 95 meters.

Kcd

Dalton Sandstone member, Crevasse Canyon Formation. Light tan-colored very finegrained quartz sandstone. Grains are mostly subrounded quartz and light gray grains that appear to be either altered feldspar or argillite grains. Some beds appear massive to very weakly bedded. Other beds show planar cross-bedding in sets up to about 30 cm thick. Forms a prominent light gray cliff. See measured sections 2 and 20 for a more detailed description. Thickness 24 meters.

Kmm

Mancos Shale, Mulatto tongue. This unit is composed mostly of very thinly bedded siltstone, and minor shale and fine-grained sandstone. The unit typically forms a steep resistant slope which contains two intervals of medium-bedded fine-grained sandstone. These two intervals form light tan cliffs composed of thin planar beds. Thickness 80 meters.

Kcdc

Dilco Coal member, Crevasse Canyon Formation. Interbedded very fine-grained, well sorted yellow quartz sandstone and dark carbonaceous shale. Thin- to medium-bedded sandstone beds are typically mottled in appearance, contain abundant horizontal burrows

up to 1 cm in diameter, and are between 0.5 and 1 meter thick. Quartz grains are subrounded. Minor biotite flakes. Low-angle planar cross-beds are locally common in sets up to 20 cm thick. Ripple marks are common on the undersides of some sandstone beds. Shale horizons are characteristically dark gray, contain abundant dark carbonaceous matter and brown to black lignite coal beds up to several tens of centimeters thick, and are between 3 and 7 meters thick. Shale beds also contain permineralizied wood fragments and leaf and seed molds. Thickness about 30 meters. Kg

Gallup Sandstone. Very fine- to fine-grained quartz sandstone Light tan-colored, fine- to very fine-grained quartz sandstone. It contains subangular to subrounded quartz grains and sparse dark lithic grains. The unit is typically horizontally bedded to massive. Planar cross-beds are common in sets up to 1-2 meters. The unit is well exposed from the southwest corner of the map to the northeast corner. It is best exposed near the mouth of Seboyeta Canyon where it forms a prominent cliff. Here, the overlying Dilco Coal member has eroded recessively so that the Gallup sandstone forms a flat bench. Near Bear Canyon the formation splits into two distinct sandstone layers separated by dark shale. Unfortunately this split cannot be measured further south because the Gallup Sandstone has been completely removed farther south. Thickness between about 15 and 60 meters.

Km

Mancos Shale. Thinly layered and laminated dark shale and quartz siltstone. Weathered surfaces are typically medium gray to light tan, but fresh surfaces are characteristically dark gray to dark grayish green. Locally contains thin very fine-grained sandstone layers a few tens of centimeters thick, septarian concretions up to 1 meter across, and one thin light gray bentonite layer 15 cm thick within the lower 1/3rd of the mapped unit within the study area. Lower part also contains one layer 1 meter thick composed of very abundant oyster fossils. Translucent to clear platy crystals of gypsum up to 10 cm long are common weathering out of slopes. Sandstone layers contain sparse but ubiquitous very fine greencolored grains of unknown composition, the same size as the quartz grains. Thickness at least 250 meters.

Kms3

Sandstone. Thin to medium bedded very fine grained quartz sandstone. Sparse darker lithic grains. Dark bivalve shell fragments up to 7 cm. Most beds appear massive. Lowangle planar cross-beds are visible in the upper 3 m. Darker lithics are locally mostly dark green in color common in dark laminae. Abundant bedding-parallel burrows above crossbeds. Forms cliff. Thickness 16 meters.

Kms2

Sandstone. Upper part is thin to medium bedded very fine grained quartz sandstone. Sparse darker lithic grains. Dark bivalve shell fragments up to 7 cm. Most beds appear massive. Low-angle planar cross-beds are visible in the upper 3 m. Darker lithics are locally mostly dark green in color, common in dark laminae. Abundant bedding-parallel burrows above cross-beds. Forms cliff. Lower parties thin to medium bedded very fine grained quartz sandstone. Sparse darker lithic grains. Dark bivalve shell fragments up to 7 cm. Most beds appear massive. Low-angle planar cross-beds are visible in the upper 3 m of lower part. Darker lithics are locally mostly dark green in color, and are most

common in dark laminae. Abundant bedding-parallel burrows above cross-beds. Forms cliff. Thickness 19 meters. Kms1

Sandstone. Very fine-grained to fine-grained quartz sandstone. Upper part is mostly massive. Abundant burrows on bed planes. Faintly to strongly laminated. Medium- to thick-bedded, with low-angle planar cross-bedding. Lower part is thin- to mediumbedded. Faint horizontal laminae. Flecks of dark organic matter. Crumbly. As mapped this unit contains beds of dark shale. Thickness 19 meters.

Kd

Dakota Sandstone (shown only in cross-section).

Jm

Morrison Formation (shown only in cross-section).

Jt

Todilto Formation (shown only in cross-section).

Je

Entrada Sandstone (shown only in cross-section).

APPENDIX 1 The following thin-section descriptions are of volcanic samples from the Seboyeta 7.5’ Quadrangle, Mount Taylor Region (by Fraser Goff)

General Comments: Locations use US Geological Survey, Seboyeta 7.5 minute topographic quadrangle, UTM NAD 27 (1982) unless otherwise noted. Textures, petrology and rock names generally follow the examples in Williams et al. (1954). Rock names use classification schemes of Le Bas et al. (1986) for volcanic rocks.

Abbreviations: Kspar = generic potassium feldspar (variety noted if possible); plag = plagioclase, qtz = quartz, bio = biotite, hbd = hornblende, opx = orthopyroxene, cpx = clinopyroxene, ol = olivine; io = generic iron ore, mt = magnetite, ilm = ilmenite, gl = glass; HT-idds = high-temperature iddingsite (mainly reddish orange hematite and maghemite); ser = sericite, chl = chlorite, cc = calcite, ep = epidote, Fe-oxide = generic low-temperature secondary iron oxide(s).

F11-48: Medium-grained olivine trachybasalt (map unit Tmotb) Location: 0276534/3897190, 7660 ft, bottom upper Bear Canyon by main Silver Dollar Rd. Relations: Overlain by QTvs, bottom of unit not exposed here Color & Texture: Gray to black, medium-grained, intersertal, slightly trachytic and vesicular Primary: Contains about 3% of small ≤1 mm ol phenocrysts in groundmass of felty, semi-aligned plag, cpx, abundant tiny ol, io and minor gl Secondary: Ol is altered to HT-idds; larger crystals have fresh core Comments: This unit is observable both up and downstream Petrographic Name: Medium-grained olivine basalt F11-51: Porphyritic trachydacite pumice (Ttdt layer in QTvs) Location: 0277885/3897419, 7700 ft, end of dirt road in overflow of “Trout Pond” Relations: 3 to 5-m-thick discontinuous bed of pumice in volcaniclastic sandstone Color & Texture: White, glassy, pumiceous, highly vesicular, and slightly porphyritic Primary: Contains